In manufacturing seamless pipes using the Mannesmann-mandrel mill process, billets, which are raw materials, are heated first in a rotary hearth type heating furnace, and then are supplied to a rolling line one after another. Specifically, each of the billets is subjected to piercing-rolling by using a piercer plug and a rolling roll in a piercing rolling mill to produce a hollow shell. Next, a mandrel bar is inserted into the hollow shell in a spit form, and the hollow shell is drawn and rolled by a mandrel mill provided with a plurality of rolling stands with the outer surface thereof being restrained by a grooved roll to reduce the wall thickness thereof to a predetermined thickness. Thereafter, the mandrel bar is removed, and the hollow shell whose wall thickness has been reduced is subjected to sizing to size to a predetermined outside diameter by using a sizing mill provided with a plurality of rolling stands, whereby a pipe is obtained. For the pipe thus rolled, the weight and length thereof are measured in an actual weighing and length measuring step, and it is determined whether the weight and length are within a predefined tolerance.
In the process in which the rolled pipes are conveyed to the actual weighing and length measuring step (including a heat treating step and a cutting step), (1) a pipe having a different material, although having the same or substantially the same weight and length, in any other production lot may coexist. Also, (2) a pipe having different weight or length, although having the same material, in any other production lot may coexist. Further, (3) although pipes are within the same production lot, the pipes may be conveyed in the improper sequential order.
In view of the recent situation in which, in addition to the need for delivering pipes that meet customer's required specifications (material, length, weight), the demand for clarifying the production history of each pipe is increasing, it is desired to accurately determine the presence of any kinds of foreign pipes as described in items (1) to (3) (a pipe from any other production lot, and a pipe conveyed in the improper sequential order within the same production lot).
Conventionally, however, as described JP2001-153843A, the component analysis and the measurement of physical property values have mainly been used to determine the material of pipe, therefore the presence of a foreign pipe. This means that, among the foreign pipes of the above items (1) to (3), the presence of foreign pipe of item (1) has mainly been determined.
As described above, since it has conventionally been determined whether or not the weight and length of pipe measured in the actual weighing and length measuring step are within the predefined tolerance, an out-of-tolerance foreign pipe could be detected even if it is of item (2) or (3).
However, to take production variations into account, the above-described tolerance should inevitably be set at a large value to some degree. Therefore, a foreign pipe of the above items (2) and (3) having a weight or length close to that of the normal pipe may not be detected.